ignore self-container when in docker-in-docker

pre_commit/languages/docker.py

@@ -8,6 +8,7 @@
 from pre_commit.hook import Hook
 from pre_commit.languages import helpers
 from pre_commit.prefix import Prefix
+from pre_commit.util import CalledProcessError
 from pre_commit.util import clean_path_on_failure
 from pre_commit.util import cmd_output_b
 
@@ -42,7 +43,11 @@ def _get_docker_path(path: str) -> str:
 
     container_id = _get_container_id()
 
-    _, out, _ = cmd_output_b('docker', 'inspect', container_id)
+    try:
+        _, out, _ = cmd_output_b('docker', 'inspect', container_id)
+    except CalledProcessError:
+        # self-container was not visible from here (perhaps docker-in-docker)
+        return path
 
     container, = json.loads(out)
     for mount in container['Mounts']:

tests/languages/docker_test.py

@@ -8,6 +8,7 @@
 import pytest
 
 from pre_commit.languages import docker
+from pre_commit.util import CalledProcessError
 
 DOCKER_CGROUP_EXAMPLE = b'''\
 12:hugetlb:/docker/c33988ec7651ebc867cb24755eaf637a6734088bc7eef59d5799293a9e5450f7
@@ -171,3 +172,10 @@ def test_get_docker_path_in_docker_windows(in_docker):
         path = r'c:\folder\test\something'
         expected = r'c:\users\user\test\something'
         assert docker._get_docker_path(path) == expected
+
+
+def test_get_docker_path_in_docker_docker_in_docker(in_docker):
+    # won't be able to discover "self" container in true docker-in-docker
+    err = CalledProcessError(1, (), 0, b'', b'')
+    with mock.patch.object(docker, 'cmd_output_b', side_effect=err):
+        assert docker._get_docker_path('/project') == '/project'